Helicopters are distinguished, inter alia, by the capability to transport or to pick up loads at points to which access is difficult. One type of transport is external load transport, in which the load is attached to the helicopter or to an external winch by means of a supporting cable harness, possibly in the form of a mesh, on one or more load hooks. However, the attachment of an external load to a helicopter changes the system dynamics and detracts from the flight characteristics. Furthermore, the flight control task of the pilot is increased by the task of monitoring the external load. The external load increases the complexity of the overall system to be flown, thus leading to an increase in the stress on the pilot and, in general, to a reduction in flight safety.
When external loads are being transported there is a risk of uncontrollable, low-frequency oscillating movements as a result of aerodynamic disturbances or control inputs. The oscillation can change to an unstable oscillating movement which, furthermore, can be instigated by pilot inputs. In the end, this can lead to pilot-induced oscillations. Since, the pilot can perceive the dynamic influence of an oscillating external load on the helicopter, but in general cannot see the external load, there is a risk, in certain cases, of the correction control inputs made by the pilot in order to damp the overall system amplifying the oscillating movement of the external load. This is because the pilot intuitively attempts to counteract the lateral force exerted on the helicopter by the oscillating external load, instead of following the load to a limited extent, in order to remove lateral forces from the external load. Severe oscillating movements of the external load endanger the overall system and its crew, since their influence on the helicopter can lead to it no longer being possible to control and stabilize the helicopter.
Furthermore, the precise positioning of external loads represents a problem, since the pilot in general does not see the load suspended under the helicopter, and the external load is in motion. Furthermore, the movement of the external load is influenced by the flight movement. The detection of the respective current positioning situation and the transmission and implementation of the control instructions imply latency times which in the end lead to the correction control for load positioning lagging behind the current situation. This is made worse because the pilot stress increases the longer it takes to position the load. The greater the pilot stress, the greater the extent to which correcting control inputs resulting from incorrect positioning are necessary, and therefore the more dynamic the development of the overall system. The greater the dynamic movement that occurs in the cable harness-external load system, the more difficult it is for the ground personnel to position and control the load and the longer the positioning process lasts, which in turn results in an increase in the pilot stress and in consequence leads to a greater susceptibility to errors. Furthermore, turbulence and restricted view increase the dynamics in the overall system.
DE 10 2008 025 298 A1 discloses a method and an apparatus for damping vertical oscillations in the case of a helicopter with an attached external load, in which oscillation sensors on the helicopter are used to detect the excitation of at least two eigen forms of vertical oscillation, and opposing forces to the vertical oscillations are introduced as a function of the phase and amplitude of the detected excitations, between the load pick-up point and the load harness, by means of an actively controlled linear actuator. This active damping system, which acts directly on the cable harness, does not have any direct effect on the closed-loop control of the attitude of the helicopter, but affects only the damping of the coupling of the cable harness and external load to the helicopter. This requires a separate damping unit, which must be matched to the helicopter suspension.
DE 22 33 938 C3, U.S. Pat. No. 3,833,189 A and U.S. Pat. No. 3,756,543 A disclose a load stabilization system which reacts, on the helicopter flight control and in which the oscillating movement of the external load is measured. The rate of change of the angle between the supporting cable and the vertical of the helicopter, and its filtered rate, are passed to the regulator as controlled variables.
Furthermore, for example, DE 10 2005 022 231 A1 and DE 10 2005 022 212 A1 disclose a method and an apparatus for displaying the orientation and the movement state of a load attached to a helicopter, in which the current position of the load is measured by sensors fitted to the load. For this purpose, the load must disadvantageously be provided with a sensor. Furthermore, a communication link must be set up between the sensor and the apparatus in the helicopter.